Answer:
a. $21 per machine hours
b. $4,855          
Explanation:
a. The computation of the plantwide predetermined overhead rate is shown below:
Plantwide predetermined overhead rate is
 = Variable overhead cost rate per machine hour + Fixed overhead cost rate per machine hour
= $2 + (fixed manufacturing overhead cost ÷ Estimated machine hours)
= $2 + ($4,275,000 ÷  225,000 machine hours)
= $2 + $19
= $21 per machine hour
b. Now the total manufacturing cost assigned is 
Particulars                                      Amount
Direct material                               $1,702        
Direct labor                                $1,221
Variable manufacturing overhead $168
(84 × $2)	
Total variable cost                        $3,091
Add: 
Fixed manufacturing overhead 
(84 × $21)                                $1,764
Total manufacturing cost assigned 
to Job P90                                    $4,855